What does the C signal function return?

Look at the signal signature: sig_t signal(int sig, sig_t func). What is that return value?

Let’s remind ourselves what signal actually does. signal takes two arguments, e.g. signal(i, h). The first argument is a signal number: an int, e.g. SIGINT which is 2 and means “interrupt program”. The second argument is a signal handler, which is a pointer to a function which takes a signal number as argument. signal(i, h) requests that the handler h be called when the process receives the interrupt i.

But signal also has a return value. In h1 = signal(i, h2), h1 is the “previous action” for interrupt i, i.e. the previously registered action. If there was no action previously registered, signal returns NULL.

An example showing how the return value depends on the previous registrations for the signal number:

#include <signal.h>
#include <assert.h>
#include <stdio.h>

void catch1(int signo) {
  printf("catch1 received signal %d\n", signo);

void catch2(int signo) {
  printf("catch2 received signal %d\n", signo);

int main(void) {
  sig_t prev_sigint_handler1 = signal(SIGINT, catch1);
  assert(prev_sigint_handler1 == NULL);

  sig_t prev_sighup_handler1 = signal(SIGHUP, catch2);
  assert(prev_sighup_handler1 == NULL);

  raise(SIGINT);  // calls catch1
  raise(SIGHUP);  // calls catch2

  // Now let's swap the handlers

  sig_t prev_sigint_handler2 = signal(SIGINT, catch2);
  assert(prev_sigint_handler2 == catch1);

  sig_t prev_sighup_handler2 = signal(SIGHUP, catch1);
  assert(prev_sighup_handler2 == catch2);

  raise(SIGINT);  // calls catch2
  raise(SIGHUP);  // calls catch1

  return 0;
% ./a.out
catch1 received signal 2
catch2 received signal 1
catch2 received signal 2
catch1 received signal 1